U.S. Pat. No. 7,244,867 describes a process for converting lower molecular weight alkanes, including methane, natural gas or ethane, propane, etc., into higher molecular weight hydrocarbons, including aromatics, by bromination to form alkyl bromides and hydrobromic acid which are then reacted over a crystalline alumino-silicate catalyst to form the higher molecular weight hydrocarbons and hydrobromic acid. Hydrobromic acid is recovered by contacting the reaction product stream with water and then converted to bromine for recycle. The higher molecular weight hydrocarbons are recovered.
In a process for producing aromatic hydrocarbons such as benzene, toluene and/or xylenes (BTX) by bromination of methane to produce monobromomethane, followed by coupling of the monobromomethane to produce aromatic hydrocarbons, the coupling reactor produces hydrogen bromide (HBr), and unintended amounts of methane, light ends (C2-5 alkanes and alkenes) and heavy ends (C9+ aromatic hydrocarbons and possibly higher carbon number nonaromatic hydrocarbons). The basic process concept includes recycle of the light ends, possibly to a separate bromination reactor, and the use of methane and heavy ends as fuel. The light ends are more easily brominated than methane and if the bromination reaction of the light ends was to be carried out in the same bromination reactor a significant portion of the light ends would be converted to higher brominated species (e.g., dibromoethane, tribromopropane, etc.). Even in a separate light ends bromination reactor, it would be impossible to effect high conversion of ethane, propane and/or butanes without over-brominating because the rate of bromination increases with increasing carbon number. Further, alkenes present in the light ends stream will be brominated to alkyl dibromides regardless of the light ends bromination configuration. The multi-brominated light end derivatives decrease bromine efficiency and are more prone to coke formation in the coupling reactor than monobromomethane. The formation of coke represents a yield loss and the necessity of frequent burning off coke increases the carbon dioxide footprint of the process and reduces process reliability. If multi-brominated light ends were separated, it would create a stream with a high concentration of compounds suspected of being considerably more toxic than monobromomethane.
It can be seen that it would be advantageous to provide an integrated process concept wherein the light ends and heavy ends could be converted into useful products. The present invention provides such an integrated process.